1. Field of the Invention
The present invention relates to a switch circuit and an integrated circuit and particularly to a circuit preferably used for a high frequency switch circuit used in a high frequency band.
2. Description of the Related Art
In recent years, with the coming of an advanced information society, communication systems such as a wireless LAN and a cellular phone using the high frequency wireless communication technology has come into practical use. To constitute such a communication system, there is a growing demand for high-performance microwave components. For example, in order to realize the communication system with the wireless LAN at a low cost, a module in which a transmitter and a receiver are integrated is under development, which still requires realization of an SPDT (Single Pole Dual Throw) switch capable of handling high power. Further, an apparatus handling a high frequency signal with very high output such as a DUPLEXER used in a base station of a third-generation cellular phone requires a switch element with high withstand voltage and low loss.
For such switches, mechanical switches such as MEMS (Micro Electro Mechanical Systems) have been conventionally used. However, because the mechanical switch wears, it is conceivable that the switch is shifted from the mechanical switch to a switch circuit using a semiconductor element having a high withstand voltage in consideration of reliability, cost, and so on. Recently, to meet the application demands, semiconductor elements are under development which are excellent in high-voltage resistance characteristics, such as an HEMT (high electron mobility transistor) using SiC (silicon carbide) or GaN (gallium nitride). Moreover, a high frequency switch circuit is also being developed, in parallel, which uses a semiconductor element with low distortion operating at a high frequency such as a microwave band and applicable in the wireless communication system such as a wireless LAN and a cellular phone.
For example, as a switch circuit used at a high frequency band, an impedance transformation-type switch circuit is known in which each path uses a configuration in which quarter-wave transmission lines are connected in cascade and an FET (field effect transistor) is connected to an interconnection between the lines (see, for example, Non-Patent Document 1). Furthermore, a switch circuit is proposed which has a balanced/unbalanced conversion function (see, for example, Patent Documents 1 and 2).
The high frequency switch circuit is required to have excellent distortion characteristics in order to meet the demand of the applications such as the wireless LAN and cellular phone. In a conventional high frequency switch circuit, however, it is difficult to achieve sufficiently low distortion characteristics.
FIG. 10 is a diagram showing a circuit configuration of a conventional high frequency switch circuit. In the high frequency switch circuit 100, a path starts from an input terminal (IN) 1 and branches out into two paths which are connected to a transmitting side output terminal (TXOUT) 2 and a receiving side output terminal (RXOUT) 3. On a transmitting side path (the path between the input terminal 1 and the transmitting side output terminal 2), transmission lines 4a and 4b each having a line length of a quarter of a wavelength (λ/4) are connected in series between the input terminal 1 and the transmitting side output terminal 2, and FETs 5a and 5b are connected in parallel to contact points between them. Similarly, on a receiving side path (the path between the input terminal 1 and the receiving side output terminal 3), transmission lines 4c and 4d each having a line length of a quarter of a wavelength are connected in cascade between the input terminal 1 and the receiving side output terminal 3, and FETs 5c and 5d are connected in parallel to contact points between them.
The FETs 5a to 5d, which function as switches, are provided to control switching between the paths in the high frequency switch circuit 100. Drains of the FETs 5a and 5b are connected to an interconnection point between the transmission lines 4a and 4b and an interconnection point between the transmission line 4b and the transmitting side output terminal 2, respectively. Drains of the FETs 5c and 5d are connected to an interconnection point between the transmission lines 4c and 4d and an interconnection point between the transmission line 4d and the receiving side output terminal 3, respectively. Further, sources of the FETs 5a to 5d are grounded respectively, and gates of the FETs 5a to 5d are connected to control terminals (CONTs) 6a to 6d. Control voltages are applied from the control terminals 6a to 6d to the gates of the FETs 5a to 5d to vary the impedance of the FETs 5a to 5d, thereby switching between paths in the high frequency switch circuit 100.
FIG. 11 is a diagram showing an equivalent circuit of the conventional high frequency switch circuit 100 shown in FIG. 10 at the time of transmission switching. At the time of transmission switching, the control voltages supplied via the control terminals 6a to 6d turn off the FETs 5a and 5b and turn on the FETs 5c and 5d (An off-capacitance value of the FET in use is Coff, and an on-resistance value is RON.)
In this event, as shown in FIG. 11, the on-resistance RON is selected to be a sufficiently small value on the receiving side path, and therefore the impedance when viewing a node N22 from a node N21 is high, so that a high frequency signal cannot reach the receiving side output terminal 3. On the other hand, the off-resistance Roff is selected to be a sufficiently small value on the transmitting side path, and therefore the impedance when viewing the ground from a node N11 is high, so that a high frequency signal reaches the transmitting side output terminal 2 without loss.
FIG. 12 is a diagram showing a schematic sectional view of an FET generally used in the prior art. In FIG. 12, numeral 120 denotes a substrate (for example, InP), 121 denotes a buffer layer, 122 denotes a (high purity) channel layer, and 123 denotes a carrier supply layer. Further, S denotes a source electrode, G denotes a gate electrode, and D denotes a drain electrode. To use a transistor with a frequency equal to or higher than the micro wave here, it is necessary to make a distance Lsg between the source electrode S and the gate electrode G and a distance Lgd between the gate electrode G and the drain electrode D sufficiently small in order to reduce the on-resistance RON.
However, if the distances Lsg and Lgd are made small, the breakdown voltage of the transistor becomes low, presenting the following problem. FIG. 13 is a chart showing variations in a gate-drain (GD) voltage Vgd of the FET 5a where a high frequency signal with high power is supplied from the transmitting side output terminal 2 when the FET 5a is in an off state, and the relation between the voltage Vgd and a current IN11 flowing from the FET 5a to the node N11 with the variations, in the high frequency switch circuit 100 shown in FIG. 10.
When the distances Lsg and Lgd are small, a breakdown voltage Vbr of the FET is small to fail to increase a control voltage VCONT to be supplied to the gate terminal via the control terminal. Therefore, as shown in FIG. 13, at the time of inputting a high frequency signal with high power, the GD voltage Vgd of the FET 5a shifts to the positive side (a broken line 130 shown in FIG. 13), so that current flows between the drain and the source (DS) of the FET 5a. 
In other words, it is preferable that in the high frequency switch circuit 100 as shown in FIG. 10, no DC-like current flows to the FET when it is in an off state, but a DC-like current flows at the time of inputting a high frequency signal with high power. This greatly deteriorates distortion characteristics of a high frequency switch circuit. Accordingly, it is impossible to obtain excellent distortion characteristics in the conventional high frequency switch circuit.
(Patent Document 1)
Japanese Patent Application Laid-open No. 2003-143033
(Patent Document 2)
Japanese Patent Application Laid-open No. 2003-142931
(Non-Patent Document 1)
Y. Ayasli, R. A. Pucel, J. L. Vorhaus, and W. Fabian, “A monolithic X-band single-pole, double-throw bi-directional GaAs FET switch”, Proc. IEEE GaAs IC Symp., paper no. 21, 1980